Such pairs of tractors are used as pull tractors or as push tractors. Pull tractors pull the web of paper from a supply container over the printing platen out of the printer and are, therefore, arranged behind the printing platen as viewed in the direction of transport. Push tractors, on the other hand, push the paper taken from the supply container via guide channels to the printing platen and push it further through guides out of the printer and are therefore arranged in front of the printing platen as seen in the direction of transport. The pushing forward of the paper entails fewer problems in the case of single sheets than in the case of multi-layer continuous webs. It can, therefore, be advantageous upon feeding paper from the bottom to the printer, to use not a push tractor but rather a pull tractor. A printer, however, is rarely equipped with both types of tractors since the presence of both types would increase the cost of manufacture. On the other hand, the construction of the printer also becomes more complicated in the event that both types of tractors are provided.
In U.S. Pat. No. 4,571,103, it has already been suggested to use only one pair of tractors which can be aligned tangentially, by a swinging and locking device, both with the top side of a cylindrical printing platen and with the bottom side of the printing platen. Depending on the position, such a swingable tractor acts as a push tractor or as a pull tractor. This solution with only one pair of tractors requires, however, the swinging and locking device. It must also be kept in mind here that the operator must adjust the pair of tractors, which is not always advantageous in the case of technically untrained personnel.
In contrast to the known technique, the present invention proceeds from a standard development of the printer which provides a fixedly installed pair of tractors as push means for edge-perforated continuous webs. The tractor pair is specifically designed so that the same tractor pair can be utilized as push tractor or as pull tractor. Thus, a printer may be provided with one or the other or both.
One object of the present invention is to thus provide a locking device or means for holding in place the pair of tractors both for the push tractor and for the pull tractor embodiment.
This object is attained in accordance with the present invention for one of the types of tractor operation or tractor arrangements by mounting each tractor acting as push means on a drive shaft and by preventing the tractor in each case from turning by means of a projection provided on the tractor frame on the side opposite the drive shaft, which projection engages in a suitable guide in the printer frame. This one-sided prevention from turning permits a simple fixing in position of the pair of tractors for push operation and nevertheless permits the transverse displacement of the tractors according to the width of paper selected.
In a further development of the invention, the projections of both tractors are held on the printer frame in a continuous guide which extends parallel to the drive shaft.
In yet a further development of the present invention the projections are provided, for locking purposes, with a plurality of teeth on the side of the projection which rests against the guide and which teeth engage in similar teeth provided on the guide itself.
The object of the present invention is also attained in accordance with the invention for the other type of tractor operation or tractor arrangement in the manner that the two tractors operating as pull means and the drive shaft, as well as a round bar which is parallel to the drive shaft, are fastened on two lateral bearing plates forming a removable unit therewith and that each of the two tractors can be tightly clamped to the round bar by means of a swingable tractor flap.
In this connection, it is furthermore advantageous for the tractor flap to have a slot lying in the plane of swing which converges in the closing direction and which in size is adapted to the round bar so that it forms a tight fit therearound.